1. Field of the Invention
The present invention relates to a process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures, and more particularly, but not by way of limitation, to an improved process and apparatus for recovering vaporized gasoline light ends and the like from a mixture thereof with air expelled from tank trucks and the like.
2. Description of the Prior Art
In handling multicomponent hydrocarbon liquids such as gasoline, kerosene and the like, air-hydrocarbon vapor mixtures are readily produced which cannot be vented directly to the atmosphere due to the resulting pollution of the environment and fire and/or explosion hazard. Consequently, a variety of processes and apparatus have been developed and used for removing hydrocarbon vapors from such air-hydrocarbon vapor mixtures whereby the remaining air can be safely vented to the atmosphere. The removed hydrocarbons are generally liquefied and recombined with the hydrocarbon liquid from which they were vaporized thereby making their recovery economically advantageous.
A process for the recovery of light mixed hydrocarbon vapors from an air-hydrocarbon mixture expelled as a result of storage breathing or loading of a vented hydrocarbon vessel is described in U.S. Pat. No. 4,066,423. In accordance with such process, the air-hydrocarbon vapor mixture from which hydrocarbons are to be removed and recovered is passed through a bed of solid adsorbent having an affinity for hydrocarbons. As the mixture passes through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed and a residue gas stream is produced which is comprised of substantially hydrocarbon-free air. While a first bed of solid adsorbent is adsorbing hydrocarbons from the mixture, a second bed of solid adsorbent having hydrocarbons adsorbed thereon is regenerated by evacuation. The completeness of the regeneration of the solid aborption beds of the process is dependent solely on the degree of vacuum produced in the beds by the vacuum pump utilized. Because vacuum pumps are incapable of achieving total vacuum, i.e., lowering the absolute pressure exerted on the bed to zero, a quantity of hydrocarbons are left adsorbed on the beds after regeneration which reduces the capacity of the beds to adsorb additional hydrocarbons and reduces the service life of the adsorbent.
The hydrocarbon-rich air-hydrocarbon mixture produced as a result of the regeneration of the bed is contacted with a liquid adsorbent whereby hydrocarbons are removed therefrom and the residue gas stream from the adsorption step is recycled to the bed through which the inlet air-hydrocarbon mixture is flowing. In accordance with the teachings of U.S. Pat. No. 4,066,423, the liquid adsorbent utilized includes liquid hydrocarbons condensed from the air-hydrocarbon vapor mixture produced in the evacuation regeneration step. More specifically, the hydrocarbon-rich air-hydrocarbon vapor mixture is cooled whereby portions of the hydrocarbons are condensed and such condensed hydrocarbons are circulated into contact with the remaining air-hydrocarbon vapor mixture whereby hydrocarbon vapors are adsorbed by the liquids.
Numerous other processes and apparatus for recoverying hydrocarbons from air-hydrocarbon vapor mixtures or otherwise treating said mixtures are disclosed in U.S. Pat. Nos. 3,897,193; 3,768,232; 3,867,111; 3,455,089; 3,543,484; and 3,776,283. In all of the prior processes which utilize solid adsorbent for removing hydrocarbons from air-hydrocarbon vapor mixtures, regeneration of the adsorbent is incomplete whereby hydrocarbons are left on the adsorbent reducing the capacity, efficiency and service life thereof. Furthermore, these processes fail to utilize a flow of lean liquid adsorbent which is divided into two portions, one of which is passed directly into intimate contact with the air-hydrocarbon mixture flowing in an adsorber and the other of which is first passed in heat exchange relationship with a seal liquid circulating through a vacuum pump and cooler, and is then passed, in a heated state, into intimate contact with the air-hydrocarbon mixture in the adsorber.
By the present invention, an improved process is provided which utilizes a first portion of a liquid absorbent first in heat exchange relationship with a pump seal liquid and then in heated, intimate hydrocarbon removing relationship with an air-hydrocarbon vapor mixture. This improved process also utilizes a second portion of the liquid adsorbent in hydrocarbon removing relationship with the air-hydrocarbon vapor mixture, but without previously passing the second portion in heat exchange relationship with another substance. The utilization of the liquid adsorbent in this manner requires less liquid adsorbent thereby making the process more efficient and more economical to carry out.